The invention relates to a projection system for a step-and-scan lithographic projection apparatus, comprising a source of EUV radiation, in which a mask pattern, present in a mask, is imaged on a substrate provided with a layer which is sensitive to EUV radiation, said projection system having a magnification M of 1/4 and a numerical aperture NA of 0.1, and comprising consecutively, from the mask side to the substrate side, a first concave mirror, a convex mirror and a second concave mirror.
The invention also relates to a step-and-scan lithographic projection apparatus comprising such a projection system.
A projection system of this type is known from U.S. Pat. No. 5,353,322 which is a continuation of U.S. Pat. No. 5,220,590. Both patents relate to projection systems with three curved mirrors for use in a lithographic projection apparatus in which extreme ultraviolet radiation, hereinafter referred to as EUV radiation, is used for imaging a mask pattern on a substrate. The EUV radiation, which is also referred to as soft X-ray radiation and has a wavelength in the range between 2 and 20 nm, has the great advantage that extremely small details, of the order of 0.1 .mu.m or less, can be satisfactorily imaged with this radiation. In other words, an imaging system in which EUV radiation is used has a very high resolving power without the numerical aperture (NA) of the system having to be extremely large, so that also the depth of focus of the system still has a reasonably large value.
The three-mirror systems described in U.S. Pat. No. 5,220,590 have such designs that the following conditions are met:
the mirror systems should have a sufficiently large free working distance, of the order of 10 to 112 mm, so that there is sufficient space for moving the substrate holder and the mask holder; PA1 the aim should be to render the so-called Petzval sum equal to zero, and PA1 the numerical aperture should be at least 0.05.
The parameters which are used in U.S. Pat. No. 5,220,590 for characterizing the possible embodiments are the magnifications m.sub.1, m.sub.2 and m.sub.3 of the first concave mirror, the convex mirror and the second concave mirror, respectively, and these embodiments are shown by way of points in an X-Y system of co-ordinates in which the magnification m.sub.2 is plotted on the X axis and the ratio m.sub.1 /m.sub.3 is plotted on the Y axis. All embodiments are intended for full-field illumination, i.e. all areas of the mask pattern are illuminated simultaneously and these areas are imaged simultaneously on an IC area of the substrate. Such an illumination is used in the lithographic projection apparatuses which are known as steppers. After a first IC area of the substrate has been illuminated in such an apparatus, the substrate holder is moved in such a way that a subsequent IC area is positioned under the mask pattern and the projection system, whereafter this area is illuminated, and so forth, until all IC areas of the substrate are illuminated with the mask pattern.
It is attempted to meet the need for ICs with a larger number of components by not only reducing the dimensions of these components but also increasing the surface of the ICs. This means that the image field of the projection system must be increased. In lithographic apparatuses in which so-called deep-UV radiation at a wavelength of, for example, 248 nm, and a projection lens system having a high NA, for example, 0.5 are used, the practically unsolvable problem of the simultaneous increase of the NA and the image field has been circumvented by changing from a stepping apparatus to a step-and-scan apparatus. In such an apparatus, a rectangular or circular segment of the mask pattern and hence also of an IC area on the substrate is illuminated each time, and the mask pattern and the substrate are moved synchronously through the illumination beam, taking the magnification of the projection system into account. A different circular segment of the mask pattern is then imaged each time on a corresponding segment of the relevant IC area. After the entire mask pattern has been imaged on an IC area in this way, the substrate holder performs a stepping movement, i.e. the beginning of a subsequent IC area is introduced into the projection beam and the mask is set to its initial position, whereafter the subsequent IC area is scan-illuminated via the mask pattern. This scan-imaging method may also be used to great advantage in a lithographic projection apparatus in which EUV radiation is used as projection radiation.
All projection systems described in U.S. Pat. No. 5,220,590 are intended for stepping apparatuses; no embodiment of a step-and-scan apparatus is described. Such an embodiment is, however, described in U.S. Pat. No. 5,353,322, namely as "lens system 80" and illustrated in FIG. 6. This three-mirror system, with three aspherical surfaces, has a sufficiently large NA of 0.1, a sufficiently small distance between the extreme mirrors, and has the advantage that the mask and the substrate are situated at the same side of the system. Since a circular segment illumination is used, a physical diaphragm can be arranged in the system. However, in accordance with U.S. Pat. No. 5,353,322, the chief ray of the object beam incident on the first concave mirror must then diverge at an angle of 7.degree. to the optical axis of the system, and the system must have a relatively large power, which is the inverse of the total focal length of the system. This focal length is equal to -620 mm, which corresponds approximately to -1/2 L, in which L is the length of the system. Due to the diverging direction of incidence, the first concave mirror must have a very large diameter and this mirror must have a very large asphericity. Asphericity of a surface is understood to mean the largest deviation of this surface with respect to the spherical surface which best fits the aspherical surface. In the system 80 described in U.S. Pat. No. 5,353,322, the first concave mirror, the convex mirror and the second concave mirror have asphericities of 420 .mu.m, 55 .mu.m and 12 .mu.m, respectively, if the asphericity is measured throughout the surface. Due to the large first mirror, with its large asphericity, the three-mirror system is difficult to manufacture for scanning projection in accordance with U.S. Pat. No. 5,353,322.